Some patients with neutropenia have antibody in their serum which reacts with their own neutrophils as well as with the neutrophils of other individuals (alloantigens). The antigens involved are frequently not identifiable as known erythrocyte, neutrophil of HLA antigens. We will study human neutrophil antigens to detect new antigenic specificities, determine their inheritance in families, and characterize the reaction of antibody and neutrophils. We will use microcapillary leukoagglutination, microgranulocytotoxicity, and serum inhibition of the hexose monophosphate shunt during phagocytosis. Normal individuals and families from the MCV twin panel typed for known red cell, neutrophil, HLA and B cell antigens will be used to define specificity.